Semiconductor devices have been forced to become smaller and thinner without loss of speed or performance. Frequently, the smaller devices must perform at even higher levels. These competing demands, size v. performance, challenge manufacturers to advance their art. Usually, a metal silicide (e.g., TiSi.sub.x, TaSi.sub.x, MoSi.sub.x, or WSi.sub.x) of 50-300 nm is applied over a polysilicon surface layer of 50-400 nm that is insulated from the immediately preceding layer by a layer of silica (SiO.sub.2) that is relatively thin, e.g., about 10-30 nm. One area of importance is in the field of etching such surfaces to impart a pattern to the surface. Importantly, etching should: (a) form the desired pattern with an anisotropic profile (i.e., generate a clean, perpendicular etched surface); (b) without contaminating the etched layers; (c) without damaging the film such as might occur by over-etching into the silica layer; and (d) with a uniform etching rate across the etched surface.
There are several types of etching systems and a plurality of etching chemistry schemes. The two most common dry etching processes are plasma etching and reactive ion etching. Of these, reactive ion etching is generally preferred for high resolution replication of photoresist patterns in electrically conductive materials. The charged nature of the ions permits a greater degree of control over an impingement pattern that is perpendicular to the mask surface. Reactive ion etching is generally performed with conditions that include: an etching gas pressure of 30-200 mTorr, an etching gas flow rate of 20-100 sccm, and either a low temperature within the range of -10.degree. C. and -120.degree. C. (see, U.S. Pat. No. 5,259,923) or a relatively high temperature within the range of 50.degree. C. to 130.degree. C. (see, U.S. Pat. No. 5,354,416).
The etching gas compositions used in the art have varied but are generally based on fluorine or chlorine chemistry. U.S. Pat. No. 5,110,408 describes a plasma etching process with a plasma made from gas of SF.sub.6, CH.sub.2 F.sub.2, and Cl.sub.2. It is said that the chlorine helps to remove deposits made from the reaction by-products and protects the silica layer by increasing the selectivity of the etching process.
U.S. Pat. No. 5,200,028 uses a similar gas composition, but with differing ratios of HBr:F* ratios, for etching the silicide layer and the polysilicon layer.
U.S. Pat. No. 5,219,485 etches the silicide layer with a mixture of HCl, BCl.sub.3, and Cl.sub.2. The volumetric flowrate ratios of HCl:BCl.sub.3 :Cl.sub.2 are within 75:(30-40):(25-40). It is taught that the ratio of BCl.sub.3 :Cl.sub.2 is at least 1:1. The polysilicon layer is etched without the BCl.sub.3. If desired, NF.sub.3 may be added to increase the etching rate. Overall, it is taught that the process should be operated to achieve an etching rate ratio (R) of silicide: polysilicon of (1-2):1, preferably about 2:1. The specification refers to removal rates on the order of 90-110 nm/min. (e.g., col. 8, line 8).
U.S. Pat. No. 5,223,085 employs plasma etching with HCl and Cl.sub.2 at a flowrate of 30-200 sccm, a pressure of 0.1-1 Pa, and 50-500 W of microwave power. The Cl.sub.2 :HCl ratio is 2:1 (i.e., a ratio of HCl:Cl.sub.2 of 0.5).
U.S. Pat. No. 5,259,923 describes the use of two, and possibly three, etching gases. The first gas is selected from F, SF.sub.6, or NF.sub.3. The optional second gas is selected from HCl, HBr, Cl.sub.2, Br, or CCl.sub.4. The third gas is a combination of the second gas and an inert gas, nitrogen, oxygen, silicon tetrachloride, or carbon monoxide. The ratio of SF.sub.6 :Cl.sub.2 is within 4:6 to 7:3 at a flowrate of 20-150 sccm, a temperature of -10.degree. C. to -120.degree. C., and a pressure of 50-150 mTorr. The etching rate with 100% chlorine gas on silica was reported as 12 nm/min. Use of 20% SF.sub.6 and 80% Cl.sub.2 increased the silica etch rate to 36 nm/min. Etching rates for WSi.sub.x was 350 nm/min at 25.degree. C. with a corresponding silica etch rate of 70 nm/min. When the temperature was reduced to -30.degree. C., the silicide etch rate dropped to 300 nm/min, and the silica etch rate fell slightly to 60 nm/min.
U.S. Pat. No. 5,354,416 similarly describes the use of fluorine, chlorine, SF.sub.6, or NF.sub.3 as etching gases that reduce reaction byproduct deposits during the etching process. Removal rates are about 300 nm/min at temperatures of -60.degree. C. to -150.degree. C. and gas pressures of 1-10 mTorr.
Unfortunately, the conventional etching gas compositions and processes seek to increase the rate of etching in order to accelerate the manufacturing process. Typical chlorine-based etch systems have an etching rate of about 500 nm/min. Such processes may have had their place when metal silicide layers were relatively thick so there was time to control the etching duration. The newer films are, however, significantly thinner. The newer metal silicide layers have a thickness of about 30-75 nm. Conventional etching processes are too fast with a low degree of selectivity for etching the silicide layer in preference to the polysilicon layer. The etch duration is too short to control with the high degree of accuracy required for devices made from modern films.
It would be useful to have a process for etching metal silicide that was selective to silicide relative to polysilicon and more controllable.